1. Field of the Invention
The present invention relates to a reading apparatus for optical recording medium recorded with optical information, particularly, relates to a reading apparatus of counting a number of tracks in response to a slice level of cross talk of RF (Radio Frequency) signals recorded in an optical recording medium while a special reproduction such as high speed image searching.
2. Description of the Related Art
Generally, while an optical disc is reproduced or recorded, some methods such as a method based upon a tracking error signal and a method based upon an RF (Radio Frequency) signal are well known as a method of counting a number of tracks while a special reproduction such as high speed image searching. In case of a method of counting a number of tracks based upon an RF signal, a signal for counting is produced by holding and detecting a top and a bottom levels of an RF signal by a predetermined time constant, then a number of tracks is counted after the top and the bottom levels are sliced by a predetermined slice level. In case of a system of processing various signals obtained by a pickup after the various signals are converted into digital, for example, in case of a system using a 5 volt single power supply, a reference voltage is set up to 2.5V for instance and a signal for counting is adapted with centering the reference voltage, then the signal for counting is digitized.
Since a signal for counting, which is produced by holding and detecting a top and a bottom levels of an RF signal as mentioned above, has amplitude of centering a certain value, a certain value can be proper for a slice level as long as optical dispersion of a pickup is low and difference in cross talk of disc by disc is small, wherein the cross talk value is a range of fluctuation of a scanning signal, which is obtained by scanning or reproducing a disc with one rotation in a circumference direction. Further, in case that a center of a signal for counting is fluctuated, the fluctuation can be absorbed by applying an AC (Alternative Current) coupling when a signal for counting produced in analog is supplied to a digital signal processing block as long as image searching is not performed at a high speed.
FIG. 1 shows a reproduction apparatus or a reading apparatus of a comparative example. In FIG. 1, a disc 1 is a CD (Compact Disc) for instance and is rotated by a spindle motor 2. Information recorded in the disc 1 are read out by a pick up (PU) 3 as a signal. A preamplifier 4 produces an RF signal, a tracking error (TE) signal, and a focus error (FE) signal in accordance with the signal read out by the PU 3. The FE and the TE signals are supplied to a driver circuit 12 for driving a focus coil and a driver circuit 13 for driving a tracking coil by way of analog to digital converters (ADC) 6 and 9, equalizer and amplifier (EQ/AMP) 7 and 10, and digital to analog converters (DAC) 8 and 11 in a servo amplifier section 5 respectively. The driver circuits 12 and 13 drive a focus coil and a tracking coil with a tracking drive signal (TRKG) and a focus drive signal (FOCS) respectively, wherein the focus coil and the tracking coil are not shown in FIG. 1.
The RF signal produced by the preamplifier 4 is supplied to a top detector (TOP) 17, a bottom detector (BTOM) 18, and a synchronous detecting circuit (SYNC) 25 respectively by way of an ADC 15 and an EQ/AMP 16 in a decoder section 14. The SYNC 25 produces a synchronous signal in accordance with the RF signal. A spindle error (SPDLERR) circuit 26 produces a spindle error (SPDL) signal in accordance with the synchronous signal, then the SPDL signal is supplied to a driver circuit 28 by way of a DAC 27. The driver circuit 28 drives the spindle motor 2 in response to the SPDL signal.
The TOP 17 and the BTOM 18 produce a top envelope signal and a bottom envelope signal of the RF signal respectively. These two envelope signals are added by an adder 19 and a signal having amplitude by track cross is produced, wherein the track cross is produced by the top and the bottom envelopes of the RF signal. The signal is sliced with a certain slice level and counted by a track counting section (COUNT) 20 while high speed image searching. A carriage error signal is produced in a carriage error circuit (CARGERR) 21 in accordance with a value of track count and is supplied to a driver circuit 23 through a DAC 22. The driver circuit 23 drives a carriage motor 24 with a carriage drive signal (CARG) in response to the carriage error signal.
In addition thereto, the TOP 17 and the BTOM 18 are arranged in a subsequent stage of the ADC 15 and the EQ/AMP 16 in FIG. 1. However, it is known that an RF signal in analog produced by the preamplifier 4 can be directly supplied to the TOP 17 and the BTOM 18 and the signal having amplitude of track cross is converted from analog to digital in between the adder 19 and the COUT 20 by the AC coupling.
In case that a plurality of data are sequentially wrote down in one CD-R (Compact Disc Recordable) by a plurality of writing apparatuses having respective pick up of optically various characteristics, there existed some problems such that a number of tracks can not be accurately counted since a value of cross talk varies by area written with information. Further, since high speed disc rotation and high speed image searching are required for a CD-ROM (Compact Disc Read Only Memory) recently, a high frequency component decreases by the AC coupling of signals for counting and a problem such that a number of track can not be accurately counted arises.
Main waveforms in process of the comparative example are depicted in FIGS. 2(a) through 2(d). In case that a tracking servo is in an open loop state, an RF signal in analog produced by the preamplifier 4 contains an amplitude component in a lower part of a waveform as shown in FIG. 2(a) due to an eccentric component of the disc 1. With assuming that a peak to peak level of RF signal is 2.0 V.sub.p-p and a lower amplitude component by an eccentric component is 0.5 V.sub.p-p, a value of cross talk is as follows: EQU (2.0-0.5).times.100/2.0=75[%]
A top envelope signal, which is hereinafter called a top component, and a bottom envelope signal, which is hereinafter called a bottom component, respectively produce by the TOP 17 and the BTOM 18 in accordance with the RF signal, are shown in FIG. 2(b) as analog signals for easier explanation. An amplitude component by an eccentric component is extracted as shown in FIG. 2(c) by adding or subtracting these two signals in the adder 19.
In case that a reference voltage of a signal inputted to the preamplifier 4 is 2.5V and an average value of an RF signal produced by the preamplifier 4 is the same voltage as that of the reference voltage as shown in FIG. 2(a), a top component is approximately 3.5V and a bottom component has a voltage range from 1.5V to 2.0V approximately, that is, an average value of the bottom component is 1.75V as shown in FIG. 2(b) as long as a time constant held by the ADC 15 is fast. If the reference voltage is assumed to be 0V, the top component is approximately 1.0V and the bottom component is approximately from -0.5V to -1.0V.
Then a sum of the top component and the bottom component is obtained. A summed signal as shown in FIG. 2(c) is as follows:
Upper limit value=(3.5-2.5)+(2.0-2.5)=0.5[V], where the value 0.5V is a value based upon the reference voltage of 2.5V, that is, the upper limit value 0.5V is equivalent to 3.0V, which is higher than the reference voltage of 2.5V by 0.5V.
Lower limit value=(3.5-2.5)+(1.5-2.5)=0[V], where the value 0V is a value based upon the reference voltage of 2.5V, that is, the lower limit value 0V is equivalent to the reference voltage of 2.5V. Therefore, the amplitude of the summed signal is from 0.5V to 0V if the reference voltage of 2.5V is assumed to be 0V. Accordingly, in case that a track count is performed while high speed image searching in accordance with the waveform for counting, a slice level is preferable to be 0.25V, which is an average value of the amplitude from 0.5V to 0V. In other words, the slice level shall be 2.75V, which is higher than the reference level of 2.5V by 0.25V.
However, since actual cross talk of an RF signal is fluctuated by uneven surface flatness or eccentricity of the disc 1, a lower amplitude component is disturbed as shown in FIG. 2(d). Therefore, there existed a problem that a track counting is not accurately performed with the above mentioned slice level.